Vehicle seat occupancy detection and/or classification systems are nowadays widely used in vehicles, in particular in passenger cars, for providing a seat occupancy signal for various appliances, for instance for the purpose of a seat belt reminder (SBR) system or an activation control for an auxiliary restraint system (ARS). Numerous seat occupancy detection and/or classification systems include seat occupancy sensors that are based on capacitive sensing, employing capacitive sensors.
The term “capacitive sensor” designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person's body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which may be identical with or different from emitting antenna electrodes—at which the influence of an object or living being on the electric field is detected.
Vehicle seat occupancy detection and/or classification systems are known to be employed as a means of assessing a potential activation of an installed vehicle passenger restraint system, such as an airbag. An output signal of the seat occupancy detection and/or classification system is usually transferred to an electronic control unit of the vehicle to serve, for instance, as a basis for a decision to deploy an air bag system to the vehicle seat.
International application WO 2011/079092 A1 describes an occupant detection system and a method for detecting an occupant seated in a vehicle seat. An electrode is arranged in a seat proximate to an expected location of an occupant for sensing an occupant proximate thereto. The electrode may be integrated with a seat heater. Control circuitry controls the seat heater. A signal generator is coupled to the electrode and configured to output to the electrode a plurality of signals at a plurality of frequencies. Occupant detection circuitry detects voltages responsive to the plurality of signals at the plurality of frequencies and detects a state of occupancy based on the detected voltages. An LC circuit coupled to the electrode and the control circuitry suppresses capacitance generated by the control circuitry.
Document EP 2 572 943 A1 discloses a combined heating and capacitive seat occupant sensing system. The system comprises a heating element which is also used as antenna electrode for a capacitive sensing circuit. The system further comprises a common choke connected to the terminals of the heating element for DC-coupling the heating element to a heating current supply. The capacitive sensing is configured to drive two alternating currents with two different frequencies into the heating element, and to determine occupancy of the seat based on calculations, taking into account the behavior of the common choke.